"The first and second bulletins, which appeared in 1874, are separately paged pamphlets...constituting a 'First Series' of the publication. Bulletins which appeared in 1875, being those of a 'Second series' and six in number, are continuously paged. With no. 6 were issued title, contents, index, & c., for all the numbers of both 'series' which had then appeared; the design being that these should together constitute volume I, in order that the inconvenient distinction of 'series' might be dropped."
Geological and Geographical Survey of the Territories (U.S.)